This study was aimed at rearing new self-compatible and Alternaria alternata resistant cultivars of Japanese pear as well as investigating the common self-incompatibility in 'Osanijisseiki' to be experimented on at Tottori Prefecture.The results of the study are summarized as follows :1. Self-compatible and Alternaria alternata resistance strains of high quality 'Nijisseiki lines were collected out of first selfed and hybrid strains.2. Assuming that self-compatibility of 'Osanijisseiki' is based on the actions of gene to inhibit response of self-compatibility in style, self-compatibility gene (Cc) is dominant in the first selfed and hybrid strains of 'Osanijisseiki' used as mother plant.The crosses between 'Kosui' and 'Osanijisseiki' and 'Shinsui' and 'Osanijisseiki' were different from the above-mentioned, assumption and therefore difficult to account for in the study. Even if self-compatibility of 'Osanijisseiki' is based on the stylar part mutation of either S2 or S4 gene, F1 segregations of the crosses between 'Kosui' and 'Osanijisseiki' and 'Shinsui' and 'Osanijisseiki' could not be explained either.In all crosses, 'Osanijisseiki' is made use of as mother plant in order to select for efficient self-compatible strains.3. In order to certify the relationship between self-incompatibility and stylar RNase, RNase activities were compared in 'Osanijisseiki' and two of its F1 strains. Self-compatibility was observed in two F1 of 'Osanijisseiki' while 'Nijisseiki' had self-incompatibility. There was no difference between them.